Heretofore, radioactive liquid solutions of iodine 131 had been employed in a wellbore followed by an overflush of a non-radioactive liquid to get the radioactive liquid out of the wellbore before nuclear logging. Thereafter, a nuclear log survey was run to determine where the liquid went when it left the wellbore and entered the surrounding formation. A problem with liquid radioactive tracer surveys has been that if the overflush is too large, the radioactive liquid can be washed so far away from the wellbore that the nuclear log survey cannot detect the liquid, i.e., the liquid is flushed beyond the range of the logging tool.
Also, heretofore, propping agents used in fracturing treatments have been radioactively tagged to determine where the propping agent went. Unfortunately, for finding the vertical extent of a fracture in a wellbore, radioactively tagged propping agents are not useful because they, being large, relatively heavy particles, tend to settle towards the lower end of the fracture. For example, in a fracture of a hundred foot height, the propping agent could settle in the lower twenty-five feet. Thus, a propping agent survey is not reliable in determining the vertical height of a fracture.
It is important and quite helpful to know the vertical extent of a fracture because, when designing a well treatment to be employed, knowledge of the vertical extent of the fracture will help determine the volume of fluid used in the subsequent well treatment. For example, if it is not known that a fracture extends for a hundred feet vertically, and it is assumed that the fracture is much smaller in height, such as fifty feet, the volume of fluid injected for subsequent treatments when designed for a fifty foot fracture will be woefully inadequate for the existing one hundred foot fracture and the subsequent well treatment quite ineffective, if effective at all.
Heretofore, temperature surveys have been employed to help determine the vertical extent of a fracture in a wellbore. These surveys inject a liquid which is at a substantially different temperature from the temperature of the fluids in the wellbore and then a temperature logging tool is run in the wellbore to locate temperature anomalies caused by fractures. However, unfortunately, temperature surveys are subject to many influences such as washed out areas in the wellbore, different pipe diameters and wall thicknesses, and the like, which make accurate interpretation of temperature surveys difficult. Also, temperature anomalies are transient and will disappear if the survey is not run promptly and prior to any fluid movement in the wellbore.
Accordingly, it is highly desirable to have another tool available to help determine the vertical extent of a fracture in a wellbore so that subsequent treatments of the well can be tailored to accomodate that fracture in an effective manner.